In an electron gun of a television tube, it is known that the intensity of the electron beam which is generated by the gun depends upon the potentials applied to the electrodes of this gun. The video modulation signal with which it is intended to vary the intensity of the beam is applied to one of the two electrodes of the gun. It should be noted that the invention relates in particular to systems in which the modulation signal is applied to the cathode. The signal applied to this electrode is provided by an amplifier and it comprises a continuous component of constant value, upon which the video modulation signal per se is superimposed. The continuous component, correctly regulated, is also called the blocking voltage, because it is this voltage which enables the production of a black image at the limit of the gray (that is to say, a cathode current whose intensity is effectively zero) whenever the video modulation signal is zero. In other words, this continuous component, which is developed by the amplifier, must be equal to the suppression voltage of the electron gun, which is precisely that voltage which is required for the image to be black but at the limit of the gray scale.
Now, it is known that the suppression voltage of the electron gun, a value which is characteristic for the gun, may gradually change over the service life of the cathode-ray tube. Furthermore, the generator means for the blocking voltage in the amplifier may exhibit drifting tendencies, and the image may become too bright or too dark. In a color television receiver, this phenomenon has serious consequences, because the slow drifting can affect the three electron guns in different proportions, which translates as an imbalance in the dosage of the three primary colors.
An automatic correction system for the blocking voltage of an electron gun has already been proposed, specifically in a prior patent application by the present applicant, permitting a periodic reevaluation of the continuous component provided by the amplifier, no matter what may be the causes of the drifting. In brief, this system comprises a feedback loop of the amplifier acting thereupon in order to correct the continuous polarization of one of its amplification stages and whose input magnitude was not different from the cathode current itself, measured during successive measurement periods during which it was supposed to be theoretically zero. The measurement or scanning period was selected at the end of the frame return in order to be able to maintain a zero modulation signal at the input of the amplifier. Because the feedback loop could not be permanently linked to the connection established between the output of the amplifier and the cathode, a commutator means was provided in order to activate this loop solely during the successive measurement periods, a memory being meanwhile connected to the control input of the amplifier in order to maintain the assigned value (defining the polarization of the appropriate stage of the amplifier) between two successive measurement periods.
This system functions satisfactorily so long as certain leakage currents in the gun are weak. However, certain cathode-ray tubes, by virtue of their structure, exhibit a permanent leakage current, particularly between the cathode and the filament, which is likely to distort the functioning of the feedback loop in stabilizing the continuous component of the amplifier to a value different from that of the blocking voltage required. Examples of the prior art include U.S. Pat. No. 3,855,614 and German DOS No. 2,702,817. The invention proposes, among other features, a new conception of the feedback loop permitting this supplementary problem to be taken into account. More precisely, the invention retains the principle of the regulation of the continuous polarization of the amplifier by means of a feedback loop including a memory as defined above, but it modifies both the structure of this feedback loop and its mode of sequential operation resulting in the periodic updating of the status of the memory.